1. Field of the Invention
The present invention relates to a thermally reversible color forming composition and a thermally reversible recording medium using the thermally reversible color forming composition. More particularly, the present invention relates to thermally reversible color forming compositions which involves a color forming reaction between an electron-donating chromophoric compound and an electron-accepting compound, and thermally reversible recording media using the thermally reversible color forming composition, by which a formation and a deletion of an image may be carried out by adjusting thermal energy applied to the composition.
2. Description of the Related Art
Thermal recording media which use a color producing reaction between an electron-donating chromophoric compound (hereinafter also referred to as a "color-producing agent" or "leuco dye") and an electron-accepting compound (hereinafter also referred to as a "color developer") in a recording layer is well known in the art. These recording media are generally used for a printer of a facsimile, a word processor, a scientific instrument and so on.
However, all of the conventional recording media which are practically used are non-reversible type recording media. That is, once a color is produced on the recording medium, it is non-reversibly fixed on the medium. Therefore, it is not possible to delete an image picture on the medium and to use the medium repeatedly.
On the other hand, a thermally reversible recording medium which can perform a reversible formation and a deletion of a colored image is disclosed in Japanese Laid-Open Patent Application No.60-193691 in which a combination of gallic acid and fluoroglucinol is used as the color developer, Japanese Laid-Open Patent Application No.61-237684 in which compounds such as phenol phthalene and thymol phthalene are used as the color developer, Japanese Laid-Open Patent Application Nos.62-138556, 62-138568 and 62-140881 in which a homogeneous mixture of a color-producing agent, a color developer and a carboxylate is contained in a recording layer, Japanese Laid-Open Patent Application No.63-173884 in which a derivative of ascorbic acid is used as a color developer, and Japanese Laid-Open Patent Application Nos.2-188293 and 2-18294 in which a salt of bis(hydroxyphenyl) acetic acid or gallic acid with higher aliphatic amine is used as a color developer.
However, these conventional thermally reversible recording media have problems such as compatibility in color-producing stability and color deletion ability, sufficient concentration of produced colors, and stability when used repeatedly. Therefore, these thermally reversible media are generally not suitable for practical use.
It is disclosed, in Japanese Laid-Open Patent Application No.5-124360, a thermally reversible color forming composition comprising an organic phosphate compound having a long-chain aliphatic hydrocarbon group, an aliphatic carboxylic acid or a phenol compound as a color developer and a leuco dye as a color producing agent. By using the above composition, it is possible to carry out a formation and deletion of colors relatively easily, and a stability of such formation and deletion of colors may be maintained during repeated use. In the above Patent Application No.5-124360, a thermally reversible recording medium using the thermally reversible color forming composition in a recording layer is also disclosed. The recording medium has an advantage, compared with other conventional recording media, in that an excellent stability of color formation, color deletion, and a concentration of produced colors may be achieved. However, its color deletion rate is rather slow for practical use. Thus, development of thermally reversible recording medium in which a colored image once formed can be deleted at a practically sufficient speed and yet provide stability so that colors can be maintained is desired.